(1) Field of the Invention
The present invention relates to a material capable of selective and efficient adsorption of uranium dissolved in aqueous solution, as well as a process for preparing said material.
(2) Description of Prior Art
In Japan where natural resources are scarce, recovery of valuable materials present in trace amounts from water-courses, sea water and mining wastewater is important not only for environmental protection but also for efficient use of resources. Among the many separating techniques so far proposed, an adsorbent made of a compound capable of forming a complex with the desired ionic specides is useful in selective separation and recovery of heavy metal salts dissolved in aqueous solutions. The material for adsorbing uranium present in a trace amount in sea water or mining wastewater must meet the following requirements: (a) high adsorption rate and capacity; (b) high selectivity for uranium adsorption; (c) usability at the temperature, pH and NaCl concentration of natural sea water; (d) low solubility in and chemical stability against sea water and desorbing liquor; (e) adaptability to cyclic use after desorption; and (f) low unit price and suitability for mass production. In an attempt at developing an adsorbent that satisfies all these requirements, a legion of inorganic and organic compounds have been studied by British, Japanese and West German scientists for their uranium adsorbing capabilities. In the earlier studies, several resins were reviewed as organic adsorbents that could be used in recovering uranium from sea water. Some resins (e.g. resorcinol acid/formaldehyde copolymer) were found to have high adsorption capacity but thereafter, they were found to be labile in sea water and gradually decompose, so researchers had no more interest in using them as uranium adsorbents. At a later time, macrocyclic hexaketones and hexacarboxylic acids were developed. In sea water, uranium is present in anionic form in a complex, so an attempt was made to recover uranium by an anion exchange resin, but no appreciable results were obtained. Noting the ability of a chelate agent to form a complex with heavy metal ions, researchers synthesized chelate resins as agents for recovering heavy metals from aqueous solutions. Among the chelates studied, those having an amidoxime group ##STR1## were found to have a particularly high selectivity for adsorbing uranium in sea water. Organic polymer compounds can be shaped more easily than inorganic compounds, but in order to use the above listed organic compounds as uranium adsorbents, they must be either supported on water-insoluble polymers or provided with a crosslinking structure, and several problems remain unsolved with this requirement. For example, macrocyclic hexaketones or hexacarboxylic acids must be supported on a polymer such as polystyrene. An amidoxime containing chelate resin is produced by first preparing a polymer containing a nitrile group and then converting said nitrile group into an amidoxime group, and conventionally, this resin has been used in the form of a copolymer with a polyfunctional monomer such as divinylbenzene in order to provide the resin with the mechanical strength and resistance to swelling necessary for using it as an adsorbent. However, depending upon the proportions of the monomers, the molecular weight of the copolymer, and the density of the bridges introduced, it often occurs that great difficulty is involved in shaping the copolymer in a desired from such as particles, fibers, web or membrane.